1. Technical Field
The invention relates to distributed data processing systems and more particularly to management of a buffer for a network adaptor receiving and/or transmitting data packets for a node of a distributed data processing system.
2. Description of the Related Art
The passing of data between a communications link and a node comprising a computer or terminal is commonly supported by buffering the data. The object of buffering is to avoid rigid synchronization in operation of the communications link and the internal operations of the node. A buffer may be implemented using memory or data storage registers, a disk drive, a delay line, or any one of a number of technologies. Use of a buffer allows for differences in the rate of flow of information or time of occurrence of events when transmitting information between the communications link and node.
The organization of data for transmission over a communications link has a number of consequences relating to efficient use of the communications link, management of transmission and receipt of the data, efficient use of memory upon receipt of the data, among other things. For example, a common way of handling organization of data is the use of packets, in which control and data elements are switched and transmitted as a composite whole. The use of packets simplifies a number of issues concerning routing.
Packets, or other types of data frames, may be variable in length. This has an advantage over fixed length frames in terms of efficient utilization of a communications link throughput capacity. The allocation of space in a buffer for the creation, transmission, copying and manipulation of variable length frames can, however, consume a considerable portion of the processing power of a node if the task is implemented in sequential software. The computation and construction of direct memory access (DMA) control blocks is a major part of the problem.
One way to reduce the computational load imposed on a receiving node by the use of variable length frames is to receive the frames into buffer locations of fixed length, which allow for any possible size of frame, including headers which may be added to the frame after the bulk data copy. A problem with this approach is that requires use of a great deal of memory to implement because of internal fragmentation of the buffer.
Buffer location chaining has also been used as an approach to the problem. However, doing so requires construction of descriptions of the chains and the need to allocate memory bandwidth for the reading of the chains.
Also known is the use of a first-in, first-out (FIFO) receive buffer where only so much location as is needed for a packet/frame is used. This approach suffers from the complication of a need for clearing memory. When a memory location is released, all of the frames preceding the released frame in time must also be released before the particular location may be reused.